This invention relates to a lead frame having a bed portion for mounting an IC chip, and more particularly to a type in which a number of square-formed dimples are indented on the rear side of the bed portion.
Heretofore, a bed portion (or island portion) has been provided in a lead frame for mounting an IC chip, and the bed portion is thereafter molded together with the IC chip by use of a synthetic resin and the like. However, due to the difference in thermal expansion coefficient between the synthetic resin and the metal forming the lead frame, thermal stresses tend to concentrate in corners of the bed portion.
In order to disperse thermal stresses for avoiding resin cracks, a number of dimples have been formed on the rear side of the bed portion. The dimples are arranged in a lattice form at intersecting points between a number of imaginary parallel lines disposed in a transverse direction, herein designated X, and a number of imaginary parallel lines disposed in a longitudinal direction herein designated Y perpendicular to X of the bed portion with an equal spacing maintained therebetween. For instance, in a case of a bed portion having a length of 7.6 mm and a width of 4.4 mm, the pitches of the dimples in both directions are selected to be 0.5 mm, and each dimple has been formed into an inverted frustum of a pyramid having a square base of 0.15 .times.0.15 mm, an opening angle of 60.degree., and a depth of 0.05 mm, each side of the dimple being disposed in parallel with the X-Y directions.
Although there is no restriction in the shape of the dimple itself, the aforementioned configuration has been selected because of easiness in production of a pressforming die.
However, in a case where the dimples are arranged such that each side of the dimple extends in parallel with X or Y axis of the base portion, the material removed at the time of forming dimples is shifted aside so as to form rising-up portions on the four sides of each dimple. The strained portions thus provided around the dimples interfere with each other and intensify the stresses in X-Y directions, and present bending deformation of the bed portion along the X-Y directions.